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[Regex] multilanguage

I want to allow
alphabetic characters
(not only the english alphabet but also other country
ie
&#233;,&#242;,&#249;,&#236; for italian
&#231; for francais
and so on )
the single quote ( ' ) ie Dall'ulma
the space ie De Andr&#232;

example

Dell'Ogli&#232; - good
De O&#231;ois - good
Wei&#223;m&#252;ller - good
Wei&#223;m&#252;ller3 - bad

It's [:alpha:], with only one square bracket each side. Also, [:alpha:] isn't supported by JavaScript it seems, only the shorter \w and \w versions (see this). Also, [:alpha:] (and \w) only includes a-z and A-Z, not the extended European things.
The way I see it, you have to include all the things you're not willing to accept, or vice-versa: